Exhaust gas treatment apparatus

ABSTRACT

There is provided an exhaust gas treatment apparatus  1   a  including: a tubular body  10  and a discharge electrode  12  disposed inside the tubular body. The tubular body  10  has a shape where an inner diameter of the tubular body  10  is gradually reduced in a predetermined range from a face  25  which contains a central point  24   x  of generation of corona discharge  24  generated by the discharge electrode  12  and which is perpendicular to the flow passage toward the downstream side  44  of the flow passage.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to an exhaust gas treatment apparatus.More specifically, the present invention relates to exhaust gastreatment apparatus capable of decreasing the number of the particulatespresent in exhaust gas by agglomerating the particulate matter containedin the exhaust gas.

There is increased need to remove particulate matter and harmfulsubstances in exhaust gas discharged from internal combustion enginessuch as automobile engines, construction machine engines; industrialmachine stationary engines and other combustion burning appliances; andthe like in consideration of influence on the environment. Inparticular, in recent years, the regulations regarding the removal ofthe particulate matter (hereinbelow sometimes referred to as “PM”)contained in exhaust gas have had a tendency to be strengthened on aglobal basis.

As an exhaust gas treatment apparatus for treating exhaust gascontaining PM as described above, there is disclosed, for example, anapparatus where PM is electrically collected by adsorbing PM onto apositively electrified body with negatively electrifying the PM by theelectrified body after the PM is agglomerated by allowing the PM tocollide against a collision guide member provided inside the flowpassage where exhaust gas passes (see, e.g., JP-A-2001-41024). The PMpassed through the positively electrified body is collected in thefilter downstream and incinerated and removed by applying current to thepositively electrified body to allow it to function as a heater.

Such an exhaust gas treatment apparatus has a defect of increase inpressure loss because of complex flow passage constitution, andmanufacturing of the apparatus is not easy. In addition, sincesufficient agglomeration effect cannot be obtained, the particulatematter passes through the apparatus and is released without beingagglomerated.

From such problems, there is disclosed an exhaust gas treatmentapparatus provided with an agglomerator which electrifies particulatematter in exhaust gas by charge by corona discharge and agglomerates theparticulate matter in an electrode collecting the charge by disposingtwo kinds of electrodes of charge emission and charge collectioncommunicating the charge by corona discharge due to the application of ahigh voltage between them as an agglomerator for agglomeratingparticulate matter (PM) in exhaust gas in an exhaust gas passage whichis formed by an exhaust pipe of an internal combustion engine and whereexhaust gas circulates in the axial direction of the exhaust pipe insuch a manner that a charge communication portion of the first electrodeis located in almost the central portion in the diametrical direction ofthe exhaust gas passage (see, e.g., JP-A-2005-320955).

In addition, as an agglomerator for exhaust gas treatment apparatus usedfor an exhaust gas treatment apparatus as described above andagglomerating the exhaust gas PM charged by corona discharge by anagglomeration portion, there is disclosed an agglomerator for an exhaustgas treatment apparatus provided with the first conductive body disposedon the downstream side of the exhaust gas stream of the electrifiedportion in the agglomeration portion with applying a voltage to thefirst conductive body to have a positive electric potential (see, e.g.,JP-A-2005-324094).

Further, there is disclosed an exhaust gas purification apparatusprovided with a PM agglomeration means generating particulate matter(agglomerated PM) having a large particle diameter by agglomerating theparticulate matter contained in exhaust gas of an engine and PM trappingmeans disposed downstream of the exhaust gas flow direction of the PMagglomeration means and trapping the agglomerated PM agglomerated by thePM agglomeration means (see, e.g., JP-A-2006-29267).

However, the aforementioned JP-A-2005-320955 discloses an exhaust gastreatment apparatus where an electrode for collecting charge is disposedon the downstream side of the flow passage. In the case that theelectrode is disposed in such a manner, particulate matter charged bycorona discharge is accelerated by the electric field and passes throughwithout being trapped by the other electrode. Therefore, in theaforementioned constitution, there is a problem that the effect inagglomerating the particulate matter is small to be almost impossible toagglomerate the particulate matter practically.

In addition, in the exhaust gas treatment apparatus described in theJP-A-2005-320955, there is a description of utilizing the electrode forcollecting charge as an inner wall face of the flow passage. In such acase, the particulate matter always moving toward the downstream side onstream of exhaust gas easily passes through the range of the electricfield. Therefore, even in such a case, there is a problem that theeffect in agglomerating the particulate matter is small to be almostimpossible to agglomerate the particulate matter practically. Inparticular, in a case that the exhaust gas flow rate is high or that thenumber of the particulates contained in exhaust gas is small, it is verydifficult to trap the particulate matter on the inner wall face of theflow passage.

In addition, in the agglomerator for an exhaust gas treatment apparatusdescribed in the JP-A-2005-324094, a high voltage is applied to thefirst conductive body constituting the agglomeration portion to draw theelectrified particulate matter. It can shorten the moving distance ofthe electrified particulate matter and has high agglomeration effect incomparison with the exhaust gas treatment apparatus disposed in theJP-A-2005-320955. However, there is a problem that constitution of theelectrode (the electrified portion and the agglomeration portion) isextremely complex to make it difficult to use it for an automobile orthe like where large vibrations and the like are applied.

In addition, a PM agglomeration means used for the exhaust gaspurification apparatus described in the JP-A-2006-29267 accelerates theparticulate matter in the exhaust gas flow direction like the exhaustgas treatment apparatus described in the JP-A-2005-320955. Therefore,there is a problem that the effect in agglomerating the particulatematter is small to be almost impossible to agglomerate the particulatematter practically.

Further, the exhaust gas treatment apparatuses described in theaforementioned JP-A-2005-320955, JP-A-2005-324094, and JP-A-2006-29267have been developed in order to treat exhaust gas containing arelatively large amount of particulate matter of a diesel engine or thelike. In the case of using them for a gasoline engine or the like havinga small number of the particulates in exhaust gas in comparison with adiesel engine or the like, the number of the particulates to beagglomerated is small, and the particle diameters of the particulatesare small. Therefore, the effect in agglomerating the particulate matteris further reduced.

In particular, a new standard by EURO 6 is supposed to be applied as anexhaust gas regulation from 2012, and there is desired the developmentof an exhaust gas treatment apparatus capable of corresponding with avehicle provided with a gasoline engine as a driving mechanism. Inparticular, since a gasoline engine has a low torque, if a filterincreasing pressure loss of exhaust gas is disposed in an exhaustsystem, knocking is easily caused to cause an engine trouble or thelike. Therefore, there is desired the development of an exhaust gastreatment apparatus provided with a mechanism which hardly charge aburden on an engine or the like.

In addition, when a filter is disposed in an exhaust system of agasoline engine to run over a long distance, deposition of ash derivedfrom components contained in a gasoline of a fuel becomes a seriousproblem. Since ash does not disappear even when high-temperatureregeneration (burning) is performed in a filter unlike the PM, cloggingis caused in the filter as a result to cause the increase in pressureloss.

SUMMARY OF THE INVENTION

The present invention has been made in order to solve the aforementionedproblems of prior art and aims to provide an exhaust gas treatmentapparatus capable of decreasing the number of the particulates containedin exhaust gas by agglomerating the particulate matter contained in theexhaust gas.

As a result of earnest studies by the present inventors in order tosolve the aforementioned problems of prior art, they found out that theproblems can be solved by allowing the tubular body to have an innerdiameter gradually reduced in a predetermined range from the centralpoint generating corona discharge toward the down stream side of theflow passage in an exhaust gas treatment apparatus where a dischargeelectrode for causing corona discharge is disposed in a tubular bodyfunctioning as a flow passage where exhaust gas passes, particulatematter is charged by the corona discharge caused by the electrode, thecharged particulate matter is trapped and agglomerated on the inner wallface of the tubular body to be bloated, and then the bloated particulatematter is scattered again; which led to the completion of the presentinvention. More specifically, according to the present invention, thefollowing exhaust gas treatment apparatuses are provided.

[1] An exhaust gas treatment apparatus comprising: a tubular bodyfunctioning as a flow passage where exhaust gas passes, and a dischargeelectrode disposed in an central portion in a cross sectionperpendicular to a flow direction of the flow passage inside the tubularbody and causing corona discharge in the vicinity thereof by applying avoltage; wherein the tubular body has a shape where an inner diameter ofthe tubular body is gradually reduced in a predetermined range from aface which contains a central point of generation of corona dischargegenerated by the discharge electrode and which is perpendicular to theflow passage toward the downstream side of the flow passage, and thenumber of particulates suspended in the exhaust gas is decreased bycharging the particulate matter contained in the exhaust gas passingthrough the tubular body by corona discharge caused by the dischargeelectrode, collecting the charged particulate matter on an inner wallface of the tubular body by the electric field generated from thedischarge electrode toward the inner wall face of the tubular body toagglomerate plural particulates, and allowing the agglomeratedparticulates to scatter again.

[2] The exhaust gas treatment apparatus according to [1], wherein thedischarge electrode has a disc-like electrode support disposedperpendicularly to the flow direction of the flow passage and aneedle-like discharger disposed perpendicularly to the electrode supportand wherein the central point of generation of corona discharge is thecentral point of a face which contains the central point of generationof corona discharge of the tubular body and which is perpendicular tothe flow passage.

[3] The exhaust gas treatment apparatus according to [2], wherein thetubular body has a shape where the inner diameter of the tubular body isreduced so that the distance from the central point of generation ofcorona discharge to the inner wall face of the tubular body in thepredetermined range toward the downstream side of the flow passage is inthe range of ±10% of a length from the central point of generation ofcorona discharge to the inner wall face of the tubular body in the facewhich contains the central point of generation of corona discharge ofthe tubular body and which is perpendicular to the flow passage.

[4] The exhaust gas treatment apparatus according to [3], wherein thetubular body has a shape where a moving velocity of the chargedparticulate matter proceeding in an exhaust gas flow direction and adrift velocity when the particulate matter is drawn to the inner wallface are taken into consideration.

[5] The exhaust gas treatment apparatus according to any one of [1] to[4], wherein the length in the predetermined range where the innerdiameter of the tubular body is gradually reduced is 0.2 to 0.9 timesthe distance from the central point of generation of corona discharge ofthe tubular body to the inner wall face of the tubular body in the faceperpendicular to the flow passage.

[6] The exhaust gas treatment apparatus according to any one of [1] to[5], wherein the discharge electrode is supported in the central portionof the flow passage by a porcelain bushing passing through the wall faceof the tubular body and extended up to the central portion in acrosssection perpendicular to the flow direction of the flow passage.

[7] The exhaust gas treatment apparatus according to [6], wherein theporcelain bushing has groove-shaped unevenness formed on the surfacethereof.

[8] The exhaust gas treatment apparatus according to any one of [1] to[7], which is disposed in an exhaust system of a vehicle provided with agasoline engine as a drive mechanism.

The exhaust gas treatment apparatus of the present invention candecrease the number of the particulates present in exhaust gas byagglomerating the particulate matter contained in exhaust gas. Inparticular, since an exhaust gas treatment apparatus of the presentinvention can decrease the number of the particulates even withoutdisposing a filter or the like causing increase in pressure loss of theexhaust system, it can suitably be used as an exhaust gas treatmentapparatus for treating exhaust gas discharged from a gasoline engine orthe like where a harmful influence is caused by disposing a filter orthe like causing increase in pressure loss.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view schematically showing an embodiment of an exhaustgas treatment apparatus of the present invention.

FIG. 2 is a plan view from the upstream side of the exhaust gastreatment apparatus shown in FIG. 1.

FIG. 3 is a cross-sectional view showing the A-A′ cross section of theexhaust gas treatment apparatus shown in FIG. 2.

FIG. 4 is an explanatory view schematically explaining the process oftreating exhaust gas by one embodiment of an exhaust gas treatmentapparatus of the present invention.

FIG. 5 is a cross-sectional view schematically showing anotherembodiment of an exhaust gas treatment apparatus of the presentinvention.

FIG. 6 is an explanatory view schematically explaining the process oftreating exhaust gas by another embodiment of an exhaust gas treatmentapparatus of the present invention.

FIG. 7 is a front view schematically showing an example of a dischargeelectrode used for an exhaust gas treatment apparatus of the presentinvention.

FIG. 8 is a side view of the discharge electrode shown in FIG. 7.

FIG. 9 is a side view schematically showing an example of a porcelainbushing used for an exhaust gas treatment apparatus of the presentinvention.

FIG. 10 is a top view of the porcelain bushing shown in FIG. 9.

FIG. 11 is a side view schematically showing another example of aporcelain bushing used for an exhaust gas treatment apparatus of thepresent invention.

FIG. 12 is a top view of the porcelain bushing shown in FIG. 11.

REFERENCE NUMERALS

1 a, 1 b: exhaust gas treatment apparatus, 10: tubular body, 10 a: innerwall face, 12: discharge electrode, 12 a: electrode support, 12 b:discharger (needle-like discharger), 16: porcelain bushing, 18, 19:voltage introduction portion, 20: exhaust gas, 22: particulate matter,22 a: particulate matter (charged particulate matter), 22 b: particulatematter (agglomerated particulate matter), 24: corona discharge, 24 x:central point of generation (central point of generation of coronadischarge), 25: face perpendicular to the flow passage (face whichcontains central point of generation of corona discharge and isperpendicular to the flow passage) 26: electric field, 32: seconddischarge electrode, 34: unevenness, 42: upstream side of the flowpassage, 43: downstream side of flow passage, D1: inner diameter beforeinner diameter of tubular body is gradually reduced, D2: inner diameterafter inner diameter of tubular body is gradually reduced, L1: length ofpredetermined range where inner diameter of tubular body is graduallyreduced (length of predetermined range), R1: distance from central pointof generation of corona discharge to inner wall face of tubular body inface perpendicular to the flow passage (radius in cross section oftubular body)

DETAILED DESCRIPTION OF THE INVENTION

Embodiments for carrying out the present invention will be describedwith referring to drawings. However, the present invention is by nomeans limited to these embodiments, and, needless to say, variouschanges, modifications, and improvement may be made on the basis ofknowledge of a person of ordinary skill in the art as long as they donot deviate from the scope of the present invention.

[1] Exhaust Gas Treatment Apparatus:

FIG. 1 is a side view schematically showing an embodiment of an exhaustgas treatment apparatus of the present invention, FIG. 2 is a plan viewfrom the upstream side of the exhaust gas treatment apparatus shown inFIG. 1, FIG. 3 is a cross-sectional view showing the A-A′ cross sectionof the exhaust gas treatment apparatus shown in FIG. 2, and FIG. 4 is anexplanatory view schematically explaining the process of treatingexhaust gas by one embodiment of an exhaust gas treatment apparatus ofthe present invention. Incidentally, FIG. 4 is a cross-sectional viewshowing an enlarged portion of a cross-sectional view shown in FIG. 3.

As shown in FIGS. 1 to 4, the exhaust gas treatment apparatus 1 a of thepresent embodiment is provided with a tubular body 10 functioning as aflow passage where exhaust gas 20 passes and a discharge electrode 12disposed in an central portion in a cross section perpendicular to aflow direction of the flow passage inside the tubular body 10.

The exhaust gas treatment apparatus 1 a of the present embodimentcharges the particulate matter 22 contained in exhaust gas 20 passingthrough the tubular body 10 by corona discharge 24 caused by theaforementioned discharge electrode 12, collects the charged particulatematter 22 a on the inner wall face 10 a of the tubular body 10 by theelectric field 26 generated from the discharge electrode 12 toward theinner wall face 10 a of the tubular body 10 to agglomerate pluralparticulates 22 a, and allows the agglomerated particulates 22 b toscatter again, thereby decreasing the number of the particulates 22suspended in the exhaust gas 20.

In the exhaust gas treatment apparatus 1 a of the present embodiment,the aforementioned tubular body 10 has a shape where an inner diameterof the tubular body 10 is gradually reduced in a predetermined rangefrom the face 25 which contains the central point 24 x of generation ofcorona discharge 24 generated by the discharge electrode 12 and which isperpendicular to the flow passage toward the downstream side 43 of theflow passage. Incidentally, in FIG. 4, the numeral 42 shows the upstreamside of the flow passage.

In the exhaust gas treatment apparatus 1 a of the present embodiment,the particulate matter 22 a drawn to the inner wall face 10 a of thetubular body 10 by the electric field 26 communicates charge by beingbrought into contact with the tubular body 10 and trapped (i.e., dustcollection) on the inner wall face 10 a of the tubular body 10. In sucha manner, the charged particulate matter 22 a is drawn to the inner wallface 10 a of the tubular body 10 in sequence. The trapped pluralparticulates are agglomerated by the Coulomb's force to form anaggregate of the plural particulates 22 b. Then, the particulates 22 bbloated by agglomeration up to a certain size has increased mass and isunable to stay (i.e. to keep being trapped) on the inner wall face 10 aof the tubular body 10 to be discharged toward the downstream side onstream of the exhaust gas 20. Thus, the apparent number of theparticulates 22 present in the exhaust gas 20 is decreased.

Thus, the exhaust gas treatment apparatus of the present embodimenttraps the particulate matter contained in the exhaust gas on the innerwall face of the tubular body by charging the particulate matter toagglomerate plural particulates to be bloated, followed by allowing theparticulate matter to scatter again, thereby decreasing the number(apparent number) of the particulate matter in the exhaust gas. Inparticular, the exhaust gas treatment apparatus of the presentembodiment can decrease the number of the particulates even withoutdisposing a filter or the like causing rise in pressure loss of theexhaust system and can agglomerate the particulate matter in a goodcondition even in the case that the number of the particulates of theexhaust gas is small. For example, not only as the apparatus fortreating exhaust gas containing particulate matter in a relatively largeamount such as a diesel engine, but also as the apparatus for treatingexhaust gas discharged from a gasoline engine, it can suitably be used.

Incidentally, conventionally, there has been proposed an exhaust gastreatment apparatus (see, e.g., the aforementioned JP-A-2005-320955)where plural particulates are agglomerated. In such an exhaust gastreatment apparatus, in the case that the electrode for collectingcharge is disposed on the downstream side of the flow passage,particulate matter charged by corona discharge is accelerated by theelectric field and passes through without being trapped by the electrodefor collecting charge. Therefore, in the aforementioned constitution,the effect in agglomerating the particulate matter is small, and it isalmost impossible to agglomerate the particulate matter practically.

In addition, in the exhaust gas treatment apparatus disclosed inJP-A-2005-320955, there has been described the usage of the electrodefor collecting charge as the inner wall face of the flow passage.However, in such a case, the particulate matter always moving toward thedownstream side on stream of exhaust gas easily passes through the rangeof the electric field before it reaches the inner wall face of the flowpassage. Therefore, also in such a case, the effect in agglomerating theparticulate matter is small, and it is almost impossible to agglomeratethe particulate matter practically. In particular, in a case that theexhaust gas flow rate is high or that the number of particulatescontained in exhaust gas is small, it is very difficult to trap theparticulate matter on the inner wall face of the flow passage.

That is, an electric field generated from the discharge electrode towardthe inner wall face of the tubular body spreads from the central point(e.g., in the case of a needle-shaped discharge electrode, the tipthereof) of generation of corona discharge in a spherical surface shape(equipotential face), and the electric field becomes weaker as thedistance from the aforementioned central point of generation increases.Therefore, the electric field tends to be weaker toward outside of thetubular body as it goes toward downstream side from the aforementionedcentral point of generation, and the efficiency to trap the particulatematter always moving on stream of exhaust gas becomes extremely low.

In the exhaust gas treatment apparatus of the present embodiment, sincethe tubular body has a shape where the inner diameter is graduallyreduced in a predetermined range from the central point of generation ofcorona discharge toward the downstream side of the flow passage, chargedparticulate matter can be trapped in a good condition on the inner wallface of the portion of the tubular body. In particular, by constitutingthe inner wall face of the tubular body to have a shape close to that ofthe electric field which spreads in an almost spherical surface shape(equipotential face) with the central point of generation of coronadischarge as the base point, the charged particulate matter can betrapped in a better condition.

Incidentally, in the discharge electrode of the exhaust gas treatmentapparatus of the present embodiment, it is preferable that the centralpoint of the generation of corona discharge generated by dischargeelectrode is disposed in a position of the central point of a faceperpendicular to the flow passage of the tubular body. This makesspecifying of the central point of generation of corona discharge easy.That is, the central point of a face perpendicular to the flow passageof the tubular body is the central point of generation of coronadischarge (or electric field). Incidentally, when the shape of thedischarge electrode is a symmetrical shape such as a rotationalsymmetric shape, by disposing the central point of the dischargeelectrode to be located in the central point of the face perpendicularto the flow passage of the tubular body, the central point of generationof corona discharge can coincide with the central point of the faceperpendicular to the flow passage of the tubular body.

Incidentally, by gradually reducing the inner diameter of the tubularbody, the efficiency of trapping particulate matter on the inner wallface of the tubular body. However, on the other hand, since the innerdiameter of the flow passage is reduced, the pressure loss of theexhaust system tends to be increased. Therefore, as shown in FIG. 4, the“length L1 of the predetermined range” where the inner diameter of thetubular body 10 is gradually reduced is preferably 0.2 to 0.9 times,more preferably 0.4 to 0.7 times, particularly preferably 0.5 to 0.6times the distance R1 from the central point 24 x of generation ofcorona discharge 24 of the tubular body 10 to the inner wall face of thetubular body in the face perpendicular to the flow passage. Such aconstitution enables to improve the efficiency to trap the particulatematter in a good condition with suppressing the increase in pressureloss to the minimum.

Incidentally, the reduction rate of the inner diameter D2 after theinner diameter of the tubular body 10 is gradually reduced (hereinbelowreferred to as the “reduction rate of the inner diameter of the tubularbody”) with respect to the inner diameter D1 before the inner diameterof the tubular body 10 is gradually reduced (i.e., inner diameter of thetubular body 10 on the upstream side of the central point 24 x ofgeneration of corona discharge 24) is preferably 0.6 to 48%, morepreferably 5 to 23%, particularly preferably 9 to 15%. Incidentally, thereduction rate of the inner diameter of the tubular body can be obtainedby the following formula (1).

Reduction rate of inner diameter of tubular body=(D1−D2)/D1×100  (1)

(wherein D1 is the inner diameter before the inner diameter of thetubular body is gradually reduced, while D2 is the inner diameter afterthe inner diameter of the tubular body is gradually reduced)

In addition, a tubular body used for the exhaust gas treatment apparatusof the present embodiment is preferably formed so that the innerdiameter is reduced to form in an almost spherical surface shape where,in a predetermined range toward the downstream side of the flow passage,the distance from the central point of generation of corona discharge tothe inner wall face of the tubular body is within the range of ±10% ofthe length in a face containing the aforementioned central point ofgeneration of corona discharge, and perpendicular to the flow passage(i.e., radius R1 in a cross section of the tubular body (see FIG. 4)).For example, when the distance is outside the range of −10%, since theinner diameter of the tubular body becomes narrow drastically, thepressure loss of the exhaust system may increase by disposing an exhaustgas treatment apparatus. On the other hand, when the distance is outsidethe range of +10%, the rate of reduction the tubular body (in otherwards, rate of narrowing of the flow passage) is too small, and theparticulate matter moving on stream of exhaust gas may easily exceed theeffective electric field range.

The charged particulate matter moving in the electric field is moving ata speed balanced with the viscosity resistance of the exhaust gas in adirection of the inner wall face of the tubular body. Incidentally, thespeed of the moving of the particulate matter in the inner wall facedirection of the tubular body is sometimes referred to as a “driftvelocity (w)”.

The tubular body used for the exhaust gas treatment apparatus of thepresent embodiment is preferably constituted to have a shape inconsideration of a moving velocity (v) of the charged particulate matterin the exhaust gas flow direction and a drift velocity (w) drawn to theinner wall face of the tubular body in a predetermined range toward thedownstream side of the flow passage. That is, an inner face shape of thetubular body is not a spherical shape in consideration of equipotentialface simply, but a shape in consideration of also a drift velocity (w)where the particulate matter is drawn to the inner wall face of thetubular body, thereby trapping the charged particulate matter in a goodcondition.

Hereinbelow, a calculation method of the aforementioned drift velocity(w) and a method for determining a shape of the tubular body inconsideration of the moving velocity (v) and the drift velocity (w) ofthe particulate matter proceeding in the exhaust gas flow direction willbe described in more detail.

The drift velocity (w) can be obtained by the following formula (2) witha charged amount (q) of the particulate matter, charged electric fieldstrength (E), gas (exhaust gas) viscosity (μ), radius (a) of particulatematter, and Cunningham correction coefficient (Cm).

[Formula 1]

w=3qE×Cm/6πμa  (2)

However, in an actual exhaust gas treatment apparatus, a corona wind ispresent in a space where the charged particulate matter (chargedparticulates) is trapped, and it is difficult to obtain a field inconsideration of space charge of the charged particulates. Therefore, itis preferable that the “apparent drift velocity (w_(d))” is calculatedfrom the experimental value (measurement value) of a dust collectionefficiency (η) for trapping (collecting) the particulate matter todetermine an ideal shape for the tubular body by employing the “apparentdrift velocity (w_(d)) as the actual drift velocity. Incidentally, theexperimental value (measurement value) of the dust collection efficiency(η) can be measured by a particle counter, for example, the electricallow pressure impactor produced by Dekati Ltd.

Incidentally, the aforementioned dust collection efficiency (η₁) isshown by the following formula (3) with the concentration (Wi) ofparticulate matter on the inlet side and the concentration (Wo) ofparticulate matter on the outlet side of the exhaust gas treatmentapparatus.

η=1−Wo/Wi  (3)

η=1−exp(−w _(d) A/Q)  (4)

(In the formula (4), A denotes an area of a dust collection electrode(i.e., area of the inner wall face of the tubular body), and Q denotes agas flow rate per unit time.)

An “apparent drift velocity (w_(d))” can be calculated by theaforementioned formula (4) and the formula (5) with the aforementionedactual experimental value (measurement value of dust collectionefficiency (μ)). Incidentally, for example, in an area (A) of a dustcollection electrode, the inner portion of the tubular body has acircular columnar shape, the area is an area of the inner wall face in arange from the tip of the discharge electrode to 200 mm, and the gasflow rate (Q) per unit time is 15916 cm³/sec (0.955 m³/min.).

In addition, the moving velocity (v) of the charged particulate matterin the exhaust gas flow direction can be calculated from theaforementioned gas flow rate (Q) per unit time.

As a method for determining the shape of the tubular body inconsideration of the moving velocity (v) in the exhaust gas flowdirection and the drift velocity (apparent drift velocity (w_(d))), atubular body shape in an about elliptic shape can be determined byadding a drift moving distance in each position in the exhaust gas flowdirection with respect to the equipotential face from the central pointof generation of corona discharge. For example, When the distance fromthe central point of generation of corona discharge to the equipotentialface is determined as R, polar coordinates (x, y) of the point R′constituting the tubular shape in consideration of the drift velocitycan be shown by the following formulae (5) and (6), and the tubularshape in consideration of the drift velocity can be determined bysuitably determining the length of the predetermined range where theinner diameter of the tubular body is gradually reduced with using theaforementioned polar coordinates (x, y) of the point R′ constituting thetubular shape in consideration of the drift velocity.

x=R cos θ  (5)

y=R sin θ+R cos θ·w _(d) /v  (6)

Incidentally, the tubular shape in consideration of the drift velocitycan be determined by stipulating the almost elliptic shape where themoving distance (drift moving distance) of the particulate matter due ofthe electric field is considered by calculating the drift movingdistance for every 0.05 mm in the pipe direction.

In the case of determining the shape of the tubular body in such amanner, it is preferable to use assumptions as described below forsimplifying the calculation.

(1) The inner wall area of the tubular body at 200 mm on the downstreamside from the tip of the discharge electrode is determined as theelectrode area (Incidentally, the tubular body is assumed to be astraight pipe (having a fixed inner diameter of the pipe)).(2) The gas flow rate (Q) per unit time and apparent drift velocity(w_(d)) are fixed regardless of the positions of the particulates in thetubular body.(3) Even in the case that plural tips of the discharge electrode arepresent, the apparent central point of the electric field (central pointof generation of corona discharge) is a central point of a faceperpendicular to the flow passage.(4) Though electric field is generated also on the upstream side of thedischarge electrode, since the particulate matter is trapped on thedownstream side of the discharge electrode, the tubular body is narrowedonly on the downstream side of the flow passage. Incidentally, the driftof the particulate matter due to the electric field generated on theupstream side of the discharge electrode is ignored.

As described above, the shape of the tubular body can be made suitablefor trapping the charged particulate matter, agglomerating the trappedparticulate matter, and scattering the bloated particulate matter due tothe agglomeration again. Further, by the use of the aforementionedassumptions, the shape of the tubular body can be determined moreeasily.

Incidentally, as a method for confirming the decrease in the number ofthe particulates in exhaust gas by the exhaust gas treatment apparatusof the present embodiment, a particle counter is disposed on thedownstream side of the exhaust gas treatment apparatus to measure thenumber of the particulates in the exhaust gas. An example of theaforementioned particle counter is the Electrical Low Pressure Impactor(hereinbelow sometimes referred to as “ELPI”) produced by Dekati Ltd.According to such ELPI, measurement (sampling) of the number ofparticulates having a particle diameter of 0.007 to 10 μm in theparticulate matter is possible. Incidentally, upon measurement, theparticulates are classified by the following particle diameters: 0.007to 0.014, 0.014 to 0.0396, 0.0396 to 0.0718, 0.0718 to 0.119, 0.119 to0.200, 0.200 to 0.315, 0.315 to 0.482, 0.482 to 0.760, 0.760 to 1.23,1.23 to 1.95, 1.95 to 3.08, 3.08 to 6.27 (unit of μm).

As a more specific measurement method, in the first place, a particlecounter is disposed on the downstream side of the exhaust gas treatmentapparatus, and the number of the particulates in the exhaust gas ismeasured (sampled) in each of the case of applying a voltage in each ofthe electrodes (upon applying a voltage) and the case of applying novoltage (upon applying no voltage). Next, from the sum of themeasurement data by each particle diameter range, the total number(total discharge number) of the particulates discharged from thedownstream side is calculated out. Next, from the data of the totaldischarge number at each of the time of applying a voltage and the timeof applying no voltage, the ratio of the number of the particulatesreduced by the exhaust gas treatment apparatus of the present embodimentcan be obtained.

In the case of treating exhaust gas by the use of an exhaust gastreatment apparatus of the present embodiment, there is no particularlimitation on the flow rate of the exhaust gas to be treated, and thenumber of the particulates can be decreased in a good condition with anexhaust gas flow rate of, for example, 200 m/second or less.Incidentally, since the exhaust gas flow rate upon running of a generalvehicle provided with a gasoline engine as a drive mechanism is 150m/second (in the case of 2 L engine, 6000 revolutions, and exhaust gastemperature of about 600° C.), even in such a vehicle, the treatment ofexhaust gas can be performed in a good condition by the use of theexhaust gas treatment apparatus of the present embodiment.

Hereinbelow, the exhaust gas treatment apparatus of the presentembodiment will be described in more detail by each of the constituents.

[1-1] Tubular Body:

The tubular body is connected to an exhaust system where exhaust gasbeing discharged from an internal combustion engine or the like andcontaining particulate matter passes to function as a flow passage whereexhaust gas passes. As described above, the tubular body is constitutedto have a shape where the inner diameter is gradually reduced in apredetermined range from the face containing the central point ofgeneration of corona discharge and perpendicular to the flow passagetoward the downstream side of the flow passage. Such a tubular body maybe connected independently to the exhaust pipe for discharging exhaustgas from the internal engine, or apart of the exhaust pipe provided onthe internal combustion engine may be used as the tubular body in theexhaust gas treatment apparatus of the present embodiment.

In the exhaust gas treatment apparatus of the present embodiment, adischarge electrode and a dust collection electrode are disposed insidethe tubular body, and inside of the tubular body is conducted atreatment where (1) the particulate matter is charged by coronadischarge, (2) the charged particulate matter is trapped on the innerwall face of the tubular body by an electric field, (3) plural trappedparticulates are agglomerated, and (4) the agglomerated particulates arescattered again.

Such a tubular body is used as not only a flow passage where exhaust gaspasses, but also the opposed electrode to generate an electric fieldbetween the tubular body and the dust collection electrode. Therefore,the tubular body is preferably constituted of a conductive material.When the tubular body is used as the opposed electrode of the dustcollection electrode or the like, the tubular body is preferablygrounded.

As the tubular body, there can suitably be used a body made of aconductive material such as stainless and iron used for an exhaust pipeof an automobile.

There is no particular limitation on the length of the tubular body aslong as the tubular body has a length where the discharge electrode isdisposed inside thereof, the range having the gradually reducing innerdiameter of the tubular body is provided as described above, and theexhaust gas treatment from the charge of the particulate matter to there-scattering of the aforementioned particulate matter can be performedin the tubular body.

In addition, the tubular body preferably has a circular cylindricalshape having a straight central axis and is preferably constituted sothat the inner diameter is reduced in the aforementioned range. Such aconstitution can trap the charged particulate matter in a good conditionand inhibit excessive rise in pressure loss.

[1-2] Discharge Electrode:

The discharge electrode is an electrode for generating corona dischargewhich charges the particulate matter in exhaust gas and disposed in thecentral portion in across section perpendicular to the flow direction ofthe flow passage inside the tubular body. In addition, the dischargeelectrode is used also as an electrode for generating an electric fieldfor trapping the charged particulate matter with the inner wall face ofthe tubular body functioning as a flow passage of a fluid as an opposedelectrode. This enabled the charged particulate matter to be trapped onthe inner wall face of a tubular body.

The discharge electrode is preferably constituted to be able to generatecorona discharge in a region up to the inner wall face of the flowpassage formed by the tubular body including the vicinity of thedischarge electrode in such a manner that more particulate matter,preferably all the particular matter in the exhaust gas passing throughthe tubular body passes through the space where the corona discharge isgenerated.

In the exhaust gas treatment apparatus 1 a shown in FIGS. 1 to 4, thedischarge electrode 12 is supported in the central portion of the flowpassage by the porcelain bushing 16 passing through the wall face of thetubular body 10 and extended to the central portion in a cross sectionperpendicular to the flow direction of the flow passage. Inside theporcelain bushing 16 is disposed a voltage introduction portion 18including a wire for applying a voltage (high voltage) to the dischargeelectrode 12, and the voltage introduction portion 18 and the dischargeelectrode 12 are electrically connected with each other in a state thatelectrical insulation between the voltage introduction portion 18 andthe tubular body 10 is secured.

Incidentally, FIGS. 1 and 4 show an example of a case where a porcelainbushing 16 is disposed on the upstream side of the central point 24 x ofgeneration of corona discharge 24 so as to pass through the wall face ofthe tubular body 10. However, the porcelain bushing 16 may be disposedso as to pass through the wall face of the tubular body 10 on thedownstream side of the central point 24 x of generation of coronadischarge 24 as in the exhaust gas purification apparatus 1 b shown inFIGS. 5 and 6. In such a case, the porcelain bushing 16 is disposed soas to pass through the portion constituted to have a shape where theinner diameter of the tubular body 10 is gradually reduced.Incidentally, the case where the porcelain bushing 16 is disposed on theupstream side of the central point 24 x of generation of coronadischarge 24 as shown in FIGS. 1 to 4 has an advantage of easyinstallation of the porcelain bushing 15 and the discharge electrode 12.Here, FIG. 5 is a cross-sectional view schematically showing anotherembodiment of an exhaust gas treatment apparatus of the presentinvention. FIG. 6 is an explanatory view schematically explaining theprocess of treating exhaust gas by another embodiment of an exhaust gastreatment apparatus of the present invention. Incidentally, FIG. 5 showsthe same cross section as that shown in FIG. 3.

There is no particular limitation on the shape of the dischargeelectrode as long as the discharge electrode has the tip portion formedat a sharp angle and corona discharge generated therein (morespecifically, in the tip portion formed at a sharp angle) by applicationof the high voltage between the discharge electrode and the inner wallface of the tubular body. In the exhaust gas treatment apparatus 1 ashown in FIGS. 1 to 4, FIGS. 7 and 8 show an example of the case wherethe discharge electrode 12 has a disk-like electrode support 12 adisposed perpendicularly to the flow direction of the flow passage and aneedle-like discharger 12 b disposed perpendicularly to the electrodesupport 12 a (i.e., in parallel with the flow direction). By such aconstitution, an electric field concentrates in the tip portion of theneedle-like discharger 12 b to cause corona discharge in a goodcondition. In addition, by the needle-like discharger 12 b, even if thetip portion is worn away in some degree, corona discharge can be causedby concentrating the electric field. Incidentally, “needle-likedischarger” means a discharger having a thin stick shape as the entireshape besides a discharger having a sharp pointed tip portion.Incidentally, in a discharge electrode 12 having such a shape, thecentral portion of the electrode support 12 a functions as a portionwhere a voltage from the voltage introduction portion 18 (see FIG. 1) isintroduced.

Here, FIG. 7 is a front view schematically showing an example of thedischarge electrode used for an exhaust gas treatment apparatus of thepresent invention. FIG. 8 is a side view of the discharge electrodeshown in FIG. 7.

Incidentally, in FIGS. 7 and 8, 12 dischargers 12 b are disposed atregular intervals on the outside on each of the faces of the electrodesupport 12 a, and four dischargers 12 b are further disposed inside thepositions of the 12 dischargers 12 b. In addition, the four discharger12 b disposed inside are longer than the 12 discharger 12 b disposedoutside. Such a constitution can cause corona discharge over a widerrange inside the tubular body to be able to charge the particulatematter in exhaust gas in a good condition. In addition, assemblage andmanufacturing of the members are easy, and, since most of theparticulate matter in exhaust gas can be passed through in the vicinityof the discharge portion, much particulate matter can be charged in agood condition.

Incidentally, in the case that plural tips of the discharge electrodeare present, for example, as shown in FIGS. 7 and 8, even in the casethat the discharge electrode 12 has a disc-shaped electrode support 12 adisposed perpendicularly to the flow direction of the flow passage andtwo or more needle-shaped dischargers 12 b disposed perpendicularly tothe electrode support 12 a, the central point of generation of coronadischarge (in other words, central point of generation of the electricfield) can be the central point of a face perpendicular to the flowpassage.

Incidentally, the shape of the discharge electrode is not limited to theaforementioned shape where needle-like dischargers are disposed on theelectrode support, and, for example, a plurality of plate-like bodieseach having at least one sharp blade edge-like side may be disposed onthe electrode support. In the case of such a discharge electrode, theelectric field concentrates on the blade edge of each plate-like body tocause corona discharge.

Regarding the material constituting the discharge electrode, there cansuitably be used the same material as that constituting an electrodehaving conventionally been used for an exhaust gas treatment apparatusperforming agglomeration by charging the particulate matter in exhaustgas. Examples of the material include stainless steel, iron, nickel,kovar, platinum, copper, gold, molybdenum, and tungsten.

In addition, the discharge electrode used for the exhaust gas treatmentapparatus of the present embodiment preferably has a shape where moresharp portions are formed in a discharger portion so that the electricfield may concentrate. In addition, it is preferable that dischargersare radially disposed from the center of the cross section of thetubular body and that it has a shape causing no decrease in pressureloss. In addition, as the discharger 12 b shown in FIGS. 7 and 8, it ispreferably constituted so that many practical discharge positions arepresent.

[1-3] Porcelain Bushing:

As described above, in the exhaust gas treatment apparatus of thepresent embodiment, it is preferable that the discharge electrodedisposed inside the tubular body is supported in the central portion ofthe flow passage by a porcelain bushing passing through the wall face ofthe tubular body, being extended up to the central portion in a crosssection perpendicular to the flow direction of the flow passage, andhaving electrical insulation. Such a constitution can cause coronadischarge in a good condition by the discharge electrode and cangenerate an electric field for trapping the particulate matter in a goodcondition.

Examples of the material for the porcelain bushing include alumina,cordierite, mullite, and glass, and alumina or the like excellent ininsulation, thermal resistance, thermal shock resistance, corrosionresistance, mechanical strength, and the like, can be used moresuitably.

Such a porcelain bushing preferably has a constitution where a creepingdischarge is not caused on the surface of the porcelain bushing uponapplying a voltage on each of the electrodes. For example, as shown inFIGS. 9 and 10, as a porcelain bushing 16 used for the exhaust gastreatment apparatus of the present embodiment, one having a groove-likeunevenness 34 formed on the surface thereof can suitably be used. Here,FIG. 9 is a side view schematically showing an example of a porcelainbushing used for an exhaust gas treatment apparatus of the presentinvention, and FIG. 10 is a top view of the porcelain bushing shown inFIG. 9.

In addition, when particulate matter such as soot adheres to theporcelain bushing, insulation breakdown may be caused between thetubular body and the discharge electrode by the particulate matteradhering to the porcelain bushing to hinder the generation of theelectric field for corona discharge or the dust collection. Therefore,for example, it may have a constitution having a heater disposed insidethe porcelain bushing so that particulate matter adheres to the surfaceof the porcelain bushing can be combusted and removed by heating theheater.

In addition, it may have a constitution where a catalyst is applied onthe surface of the porcelain bushing exposed inside the tubular body tobe able to combust and remove adhering particulate matter by the heat ofexhaust gas from an engine or the like when particulate matter adheresto the surface of the porcelain bushing. For example, as such acatalyst, an oxidation catalyst used for purification of exhaust gasdischarged from an internal combustion engine or the like can suitablybe used. Suitable examples of the oxidation catalyst include aconventionally known oxidation catalyst containing platinum (Pt),rhodium (Rh), palladium (Pd), or the like.

In addition, as shown in FIGS. 11 and 12, it may have a shape where theside portion exposed inside the tubular body of the porcelain bushing 16protrudes toward the upstream side of the flow passage. A porcelainbushing 16 thus constituted hardly hinders the flow of exhaust gas toreduce resistance of exhaust gas against the porcelain bushing 16, andthe particulate matter hardly adheres to the surface of the porcelainbushing 16. Here, FIG. 11 is a side view schematically showing anotherexample of a porcelain bushing used for an exhaust gas treatmentapparatus of the present invention, and FIG. 12 is a top view of theporcelain bushing shown in FIG. 11.

[1-4] Voltage Introduction Portion:

The voltage introduction portion is a member including a wire or thelike for applying a voltage to the discharge electrode and connected toa power source (not illustrated) for causing corona discharge andgenerating an electric field for trapping the charged particulatematter. Incidentally, in the exhaust gas treatment apparatus 1 a of thepresent embodiment shown in FIGS. 1 to 4, it passes through theporcelain bushing 16 passing through the wall face of the tubular body10 from the outside of the tubular body 10 and extended up to thecentral portion in a cross section perpendicular to the flow directionof the flow passage to be electrically connected to the dischargeelectrode 12 disposed inside the tubular body 10.

[1-5] Power Source:

The powder source is for applying a voltage to the discharge electrode,and, for example, a direct current power source (DC power source), apulse power source, or the like may suitably be used. In particular, inthe exhaust gas treatment apparatus of the present embodiment, a directcurrent power source (DC power source) is preferable.

Specific values of the voltage applied on the discharge electrode andthe electric power can suitably be determined so that suitable dischargeand electric field can be generated depending on the size of the tubularbody functioning as the flow passage; the flow amount and flow rate ofthe exhaust gas passing through the flow passage; the amount, size,number of the particulates contained in the exhaust gas; and the like.

For example, though the voltage is not particularly limited, in the casethat the discharge electrode is electrically connected with each otherand that the exhaust gas treatment apparatus of the present embodimentis used for treating exhaust gas discharged from a gasoline engine, thevoltage is preferably 6 to 10 kV, more preferably 8 to 9 kV. Inaddition, the electric power is preferably 2 to 30 W, more preferably 4to 15 W. Such a constitution enables to perform the treatment of exhaustgas discharged from a gasoline engine in a good condition.

Example

Hereinbelow, the present invention will be described more specificallyby Examples. However, the present invention is by no means limited tothese Examples.

Example 1

There was manufactured an exhaust gas treatment apparatus 1 a as shownin FIGS. 1 to 3. The tubular body 10 had a circular cylindrical shapehaving a length of 300 mm in the exhaust gas flow direction, an outerdiameter of 60.5 mm, and an inner diameter of 53.5 mm, and the materialof the tubular body was stainless steel.

An alumina porcelain bushing 16 was disposed in the position of 30 mmfrom the end face on the upstream side of the flow direction of thetubular body 10 so that it passed through the tubular body 10, and avoltage introduction portion 18 was disposed inside the porcelainbushing 16. A discharge electrode 12 was connected to the voltageintroduction portion 18 to fix the discharge electrode 12 inside thetubular body 10.

As shown in FIGS. 7 and 8, the discharge electrode 12 was constituted bythe disc-like electrode support 12 a and the 16 dischargers 12 b (12dischargers at an angle of 30° outside, and 4 dischargers at an angle of90° inside) disposed on the disc-like electric support 12 a.

The disc-like electrode support had a shape where ¼ circles having aradius of 7 mm was gouged out of the ring-shaped support having theouter periphery of 20 mm so that a cross-shaped support having a widthof 3 mm remains in the central portion. In addition, through-holes eachhaving a diameter of 1.5 mm were formed in portions where thedischargers were to be disposed, and the dischargers were disposed inthe through-holes. Incidentally, the electrode support was formed ofstainless steel.

Each of the dischargers had a diameter of 1.5 mm with a sharpneedle-like tip end and was formed of stainless steel. Each of the 12dischargers disposed on the outside had a length of protruding by 10 mmfrom the surface of the electrode support, and each of the 4 dischargersdisposed on the inside had a length of protruding by 20 mm from thesurface of the electrode support.

In addition, the tubular body was formed to have a shape where the innerdiameter is gradually reduced in the range of 15 mm from the facecontaining the central point of generation of corona discharge towardthe downstream side of the flow passage. Incidentally, the shape of thetubular body was determined by a method for determining the shape of thetubular body in consideration of the aforementioned moving velocity inthe exhaust gas flow direction and apparent drift velocity.

As the measurement of the dust collection efficiency (η), exhaust gasdischarged from an automobile engine was sent in the exhaust gastreatment apparatus constituted in the same manner as in theaforementioned exhaust gas treatment apparatus of Example 1, and aconstant voltage of 8 kV (electric current of 0.5 mA) was applied to thedischarge electrode to measure the number of the particulates on boththe inlet and outlet sides of the exhaust gas treatment apparatus.Incidentally, the dust collection efficiency (η) was 0.735.

The conditions for exhaust gas in the measurement of the dust collectionefficiency (η) were as follows:

Engine rotational frequency: 2430 rpm,

Torque: 30 Nm,

Exhaust gas temperature: 339° C.,

Temperature conversion air amount: 0.955 m³/min.

In addition, as the conditions for determining the shape of the tubularbody, the area (A) of an electrode for collecting dust was the area(335.98 cm²) of the inner wall face in the range from the tip of thedischarge electrode to the position of 200 mm, and the gas flow rate (Q)per unit time was 15916 cm³/sec (0.955 m³/min).

The “apparent drift velocity (w_(d))” calculated from the aforementionedformulae (4) and (5) was 63 cm/sec. In addition, the “moving velocity(v) of the charged particulate matter proceeding in the exhaust gas flowdirection” calculated from the gas flow rate (Q) per unit time is 708cm/sec. From the above results, there was determined the shape of thetubular body in consideration of the moving velocity (v) in the exhaustgas flow direction and the apparent drift velocity (w_(d)).

The thus constituted exhaust gas treatment apparatus of Example 1 wasattached to a soot generator generating particulate matter by a burnerwith light oil being used as the fuel, and test exhaust gas (hereinbelowreferred to as “exhaust gas” simply) at about 195° C. was introduced ata flow rate of 1.5 m³/min. In such a state, as shown in Table 1, adirect current voltage of 8 kV with an electric power of 5 W was appliedon the discharge electrode of the exhaust gas treatment apparatus ofExample 1 to treat the exhaust gas. The mass (g/hour) of the particulatematter on the inlet side of the exhaust gas treatment apparatus duringtreating the exhaust gas, the number (×10⁷ particulates/sec.), the mass(g/hour), and the average particle diameter (μm) of the particulates onthe outlet side were measured. The measurement results are shown inTable 2.

(Measurement of the Number of Particulates)

A particle counter (Electrical Low Pressure Impactor (ELPI) produced byDekati Ltd.) was equipped on the downstream side of the dischargeelectrode to measure the number of the discharged particulates by eachparticle diameter range on the downstream side of each electrode in thecase that a voltage was applied on the discharge electrode (uponapplying a voltage) and in the case that no voltage was applied (uponapplying no voltage). Then, from the sum of the measurement data foreach particle diameter range, the total number (total discharge number)of the particulate matter discharged from the downstream side wascalculated out. Incidentally, in the measurement, particles having thediameter of 0.007 to 10 μm were measured and classified into theparticulate diameter regions of 0.007 to 0.014, 0.014 to 0.0396, 0.0396to 0.0718, 0.0718 to 0.119, 0.119 to 0.200, 0.200 to 0.315, 0.315 to0.482, 0.0482 to 0.760, 0.760 to 1.23, 1.23 to 1.95, 1.95 to 3.08, 3.08to 6.27 (unit of μm). Incidentally, for example, in the case of theparticle diameter of “0.07 to 0.014”, particles having a particlediameter of 0.007 μm or more and below 0.014 μm are included.

(Measurement of Ratio of Reduced Particulate Matter)

From the data of the total discharge number (×10⁷ particulates/sec.)upon each of the application of a voltage and the application of novoltage obtained by the aforementioned measurement of the number ofparticulates, the ratio of the reduced number of the discharged particleby the use of the exhaust gas treatment apparatus was calculated by thefollowing formula (6).

(Total number upon applying no voltage−total

number upon applying a voltage)/total number upon

applying no voltage×100  (6)

(Measurement of Mass of Particulate Matter)

A bypass line from the exhaust gas flow passage was provided on each ofthe upstream side and the downstream side of the position where theexhaust gas treatment apparatus was disposed, and a paper filter forsampling the particulate matter in the exhaust gas passing through thebypass lines was disposed in each of the bypass lines. The sampling timeof the particulate matter by the paper filter was three minutes, and thechange in the paper filter mass by the sampling was calculated from thepaper filter mass before sampling weighed in advance. By the mass changein the mass of each of the paper filters disposed on the upstream sideand the downstream side, the mass (g/hour) of the particulate matter oneach of the inlet, side and the outlet side of the exhaust gas treatmentapparatus was calculated.

(Average Particle Diameter of Particulate Matter)

From the measurement data by particle diameter obtained by theaforementioned measurement of the number of particulates, the averageparticle diameter of the particulates contained in exhaust gas wascalculated by the following formula (7).

Average particle diameter=[Σ{(average particle

diameter in each sampling range)×(number of

particulates sampled in each sampling range)]/total

discharge number  (7)

(Pressure Loss Δp)

A socket was disposed as a slot for taking out exhaust gas pressure oneach of the upstream side and the downstream side on the exhaust gastreatment apparatus mounted in the exhaust gas pipe and connected with ameter (digital manometer produced by Cosmo Instruments, Co., Ltd.) bymeans of a SUS tube and a teflon tube. The pressure P1 on the upstreamside of the exhaust gas treatment apparatus and the pressure P2 on thedownstream side of the exhaust gas treatment apparatus were measured,and the pressure loss (ΔP(kPa)=P1−P2) was calculated.

TABLE 1 Distance where Inner diameter after inner Inner diameter ofinner diameter is body diameter of tubular Discharge electrode tubularbody gradually reduced is gradually reduced Addition of drift VoltageElectric power (mm) (mm) (mm) velocity (kV) (W) Example 1 53.5 15 47Added 8 16 Example 2 53.5 20 39 Added 8 16 Example 3 53.5 25 23.4 Added8 16 Example 4 53.5 15 44.3 Not added 8 16 Example 5 53.5 20 35.5 Notadded 8 16 Comp. Ex. 1 53.5 Zero 53.5 None 8 16

TABLE 2 Particulate matter Particulate matter on outlet side Ratio ofreduced Condition of exhaust gas on inlet side Average particle numberof Flow rate Temperature Mass Mass Number diameter particulates Pressureloss (m/sec.) (° C.) (g/hour) (g/hour) (×10⁷/sec.) (μm) (%) ΔP (kPa)Example 1 12 195 1.05 0.37 5.13 0.038 66 0.16 Example 2 12 195 1.07 0.374.82 0.038 68 0.4 Example 3 12 195 1.06 0.35 3.22 0.038 79 2.65 Example4 12 195 1.06 0.37 5.03 0.038 67 0.21 Example 5 12 195 1.07 0.37 4.720.038 69 0.6 Comp. Ex. 1 12 195 1.08 0.39 6.03 0.038 60 0.03

Example 2

There was manufactured an exhaust gas treatment apparatus constituted inthe same manner as in Example 1 except that the inner diameter of thetubular body was formed to be generally reduced in the range from theface containing the central point of generation of corona discharge tothe position of 20 mm toward the downstream side of the flow passage,and exhaust gas was treated in the same manner as in Example 1. Themeasurement results of the mass (g/hour) of particulate matter on theinlet side of the exhaust gas treatment apparatus and the number (×10⁷particulates/sec.), mass (g/hour), and average particle diameter (μm) ofparticulates on the outlet side during the exhaust gas treatment areshown in Table 2.

Example 3

There was manufactured an exhaust gas treatment apparatus constituted inthe same manner as in Example 1 except that the inner diameter of thetubular body was formed to be generally reduced in the range from theface containing the central point of generation of corona discharge tothe position of 25 mm toward the downstream side of the flow passage,and exhaust gas was treated in the same manner as in Example 1.

Example 4

There was manufactured an exhaust gas treatment apparatus constituted inthe same manner as in Example 1 except that the inner diameter of thetubular body was formed to be generally reduced in a spherical surfaceshape along an equipotential face in the range of 15 mm toward thedownstream side of the flow passage without adding the aforementionedapparent drift velocity, and exhaust gas was treated in the same manneras in Example 1.

Example 5

There was manufactured an exhaust gas treatment apparatus constituted inthe same manner as in Example 4 except that the inner diameter of thetubular body was formed to be generally reduced in the range from theface containing the central point of generation of corona discharge tothe position of 20 mm toward the downstream side of the flow passage,and exhaust gas was treated in the same manner as in Example 1.

In the exhaust gas treatment apparatuses of Examples 2 to 5, themeasurement results of the mass (g/hour) of particulate matter on theinlet side of the exhaust gas treatment apparatus and the number (×10⁷particulates/sec.), mass (g/hour, and average particle diameter (μm) ofparticulates on the outlet side during the exhaust gas treatment in thesame manner as in Example 1 are shown in Table 2.

Comparative Example 1

There was manufactured an exhaust gas treatment apparatus constituted inthe same manner as in Example 1 except that the tubular body had acircular cylindrical shape having a fixed size of 53.5 mm from the inletside to the outlet side of the apparatus, and exhaust gas was treated inthe same manner as in Example 1. The measurement results of the mass(g/hour) of particulate matter on the inlet side of the exhaust gastreatment apparatus and the number (×10⁷ particulates/sec.), mass(g/hour, and average particle diameter (μm) of particulates on theoutlet side during the exhaust gas treatment are shown in Table 2.

DISCUSSION

Each of the exhaust gas treatment apparatuses of Example 1 to 5 had ahigh ratio of the reduced number of particulates in comparison with theexhaust gas treatment apparatus of Comparative Example 1, and theparticulates could be agglomerated in a good condition in exhaust gas.Incidentally, since each of the exhaust gas treatment apparatuses ofExamples 1 to 5 was constituted to have a shape where the inner diameterof the tubular body was reduced in the range from the central point ofgeneration of corona discharge to the position of 15 mm, 20 mm, or 25mm, the pressure loss was increased in comparison with the exhaust gastreatment apparatus of Comparative Example 1. However, the increase wassmall, and, even if it is installed in an exhaust system of a vehicleprovided with a gasoline engine, exhaust gas can be treated in a goodcondition.

In addition, in the exhaust gas treatment apparatuses of Examples 1 to3, by the tubular body where a drift velocity was added, a sufficienteffect of reduction in the number could be obtained though the increasein pressure loss was minute. That is, by adding the drift velocity, theincrease in pressure loss could be suppressed to be very small withrespect to the effect in further reducing the number of particulates,and thereby high purification performance and reduction of strain to anengine or the like could be successfully combined further.

Incidentally, in the results shown in Table 2, the average particlediameter of the particulates on the outlet side of Examples 1 to 5 andComparative Example 1 was 0.038 μm. However, from the results ofmeasurement of the number for each particle size using theaforementioned ELPI, in exhaust gas treatment apparatuses of Examples 1to 5, it was confirmed that the number of particulates having arelatively large average particle diameter among the particulates on theoutlet side increased with the passage of time. Here, Table 3 shows thechange of the number of the particulates for each particle diameter onthe outlet side. Table 3 shows results of measurement where theparticles measured on the outlet side of the exhaust gas treatmentapparatus of Example 1 were classified into three particle diameterranges (0.007 to 0.014 μm, 0.014 to 1.23 μm, and 1.23 to 6.27 μm) tomeasure the number of particulates contained in each particle diameterrange. Incidentally, Table 3 shows the measurement results (beforeapplication) for 10 seconds before a voltage is applied and themeasurement results (7 minutes after application) for 10 seconds fromwhen 7 minutes passed after a voltage is applied.

TABLE 3 Particle diameter (μm) 0.007 to 0.014 0.014 to 1.23 1.23 to 6.27Number of Before 5.43 × 10⁸ 1.01 × 10⁹ 2.86 × 10⁴ particulateapplication matter 7 min, after 2.82 × 10⁸ 5.69 × 10⁸ 2.01 × 10⁵application

As shown in Table 3, in the results of the measurement when 7 minutespassed after application, the number of the particulates having smallparticle diameters in the range of 0.007 to 0.014 μm was reduced, whilethe number of the particulates having large particle diameters in therange of 1.23 to 6.27 μm was increased. By this, it was confirmed that,by the exhaust gas treatment apparatus of Example 1, plurality ofparticulates in exhaust gas, particularly, particulates havingrelatively small particle diameters were agglomerated to be bloated asparticulates having larger particle diameters.

INDUSTRIAL APPLICABILITY

An exhaust gas treatment apparatus of the present invention can be usedas an exhaust gas treatment apparatus decreasing the number of theparticulates present in exhaust gas by agglomerating and bloating theparticulate matter in the exhaust gas discharged from internalcombustion engines such as automobile engines, construction machineengines; industrial machine stationary engine and other combustionburning appliances.

1. An exhaust gas treatment apparatus comprising: a tubular bodyfunctioning as a flow passage where exhaust gas passes, and a dischargeelectrode disposed in an central portion in a cross sectionperpendicular to a flow direction of the flow passage inside the tubularbody and causing corona discharge in the vicinity thereof by applying avoltage; wherein the tubular body has a shape where an inner diameter ofthe tubular body is gradually reduced in a predetermined range from aface which contains a central point of generation of corona dischargegenerated by the discharge electrode and which is perpendicular to theflow passage toward the downstream side of the flow passage, and thenumber of particulates suspended in the exhaust gas is decreased bycharging the particulate matter contained in the exhaust gas passingthrough the tubular body by corona discharge caused by the dischargeelectrode, collecting the charged particulate matter on an inner wallface of the tubular body by the electric field generated from thedischarge electrode toward the inner wall face of the tubular body toagglomerate the plural particulates, and allowing the agglomeratedparticulates to scatter again.
 2. The exhaust gas treatment apparatusaccording to claim 1, wherein the discharge electrode has a disc-likeelectrode support disposed perpendicularly to the flow direction of theflow passage and a needle-like discharger disposed perpendicularly tothe electrode support and wherein the central point of generation ofcorona discharge is the central point of a face which contains thecentral point of generation of corona discharge of the tubular body andwhich is perpendicular to the flow passage.
 3. The exhaust gas treatmentapparatus according to claim 2, wherein the tubular body has a shapewhere the inner diameter of the tubular body is reduced so that thedistance from the central point of generation of corona discharge to theinner wall face of the tubular body in the predetermined range towardthe downstream side of the flow passage is in the range of ±10% of alength from the central point of generation of corona discharge to theinner wall face of the tubular body in the face which contains thecentral point of generation of corona discharge of the tubular body andwhich is perpendicular to the flow passage.
 4. The exhaust gas treatmentapparatus according to claim 3, wherein the tubular body has a shapewhere a moving velocity of the charged particulate matter proceeding inan exhaust gas flow direction and a drift velocity when the particulatematter is drawn to the inner wall face are taken into consideration. 5.The exhaust gas treatment apparatus according to claim 1, wherein thelength in the predetermined range where the inner diameter of thetubular body is gradually reduced is 0.2 to 0.9 times the distance fromthe central point of generation of corona discharge of the tubular bodyto the inner wall face of the tubular body in the face perpendicular tothe flow passage.
 6. The exhaust gas treatment apparatus according toclaim 2, wherein the length in the predetermined range where the innerdiameter of the tubular body is gradually reduced is 0.2 to 0.9 timesthe distance from the central point of generation of corona discharge ofthe tubular body to the inner wall face of the tubular body in the faceperpendicular to the flow passage.
 7. The exhaust gas treatmentapparatus according to claim 3, wherein the length in the predeterminedrange where the inner diameter of the tubular body is gradually reducedis 0.2 to 0.9 times the distance from the central point of generation ofcorona discharge of the tubular body to the inner wall face of thetubular body in the face perpendicular to the flow passage.
 8. Theexhaust gas treatment apparatus according to claim 4, wherein the lengthin the predetermined range where the inner diameter of the tubular bodyis gradually reduced is 0.2 to 0.9 times the distance from the centralpoint of generation of corona discharge of the tubular body to the innerwall face of the tubular body in the face perpendicular to the flowpassage.
 9. The exhaust gas treatment apparatus according to claim 1,wherein the discharge electrode is supported in the central portion ofthe flow passage by a porcelain bushing passing through the wall face ofthe tubular body and extended up to the central portion in a crosssection perpendicular to the flow direction of the flow passage.
 10. Theexhaust gas treatment apparatus according to claim 2, wherein thedischarge electrode is supported in the central portion of the flowpassage by a porcelain bushing passing through the wall face of thetubular body and extended up to the central portion in a cross sectionperpendicular to the flow direction of the flow passage.
 11. The exhaustgas treatment apparatus according to claim 3, wherein the dischargeelectrode is supported in the central portion of the flow passage by aporcelain bushing passing through the wall face of the tubular body andextended up to the central portion in a cross section perpendicular tothe flow direction of the flow passage.
 12. The exhaust gas treatmentapparatus according to claim 4, wherein the discharge electrode issupported in the central portion of the flow passage by a porcelainbushing passing through the wall face of the tubular body and extendedup to the central portion in a cross section perpendicular to the flowdirection of the flow passage.
 13. The exhaust gas treatment apparatusaccording to claim 5, wherein the discharge electrode is supported inthe central portion of the flow passage by a porcelain bushing passingthrough the wall face of the tubular body and extended up to the centralportion in a cross section perpendicular to the flow direction of theflow passage.
 14. The exhaust gas treatment apparatus according to claim6, wherein the discharge electrode is supported in the central portionof the flow passage by a porcelain bushing passing through the wall faceof the tubular body and extended up to the central portion in a crosssection perpendicular to the flow direction of the flow passage.
 15. Theexhaust gas treatment apparatus according to claim 7, wherein thedischarge electrode is supported in the central portion of the flowpassage by a porcelain bushing passing through the wall face of thetubular body and extended up to the central portion in a cross sectionperpendicular to the flow direction of the flow passage.
 16. The exhaustgas treatment apparatus according to claim 8, wherein the dischargeelectrode is supported in the central portion of the flow passage by aporcelain bushing passing through the wall face of the tubular body andextended up to the central portion in a cross section perpendicular tothe flow direction of the flow passage.
 17. The exhaust gas treatmentapparatus according to claim 9, wherein the porcelain in bushing hasgroove-shaped unevenness formed on the surface thereof.
 18. The exhaustgas treatment apparatus according to claim 10, wherein the porcelainbushing has groove-shaped unevenness formed on the surface thereof. 19.The exhaust gas treatment apparatus according to claim 11, wherein theporcelain bushing has groove-shaped unevenness formed on the surfacethereof.
 20. The exhaust gas treatment apparatus according to claim 1,which is disposed in an exhaust system of a vehicle provided with agasoline engine as a drive mechanism.